Coastal Hazards
STORM SURGES
Anton Micallef
Euro-Mediterranean Centre on Insular Coastal Dynamics
International Environment Institute
What is a storm surge hazard?
Ocean water levels elevated above expected astronomical
tides - caused by a passing storm:
¾ Drop of atmospheric press causes sea water to rise
by ~ 1cm for each 1 mb. drop in pressure.
+
¾ Wind stress pushing water towards the coast.
Storm surges
coastal erosion, flooding and death.
Contribution of wind strength & direction to elevating
sea surface:
¾ Inshore vs offshore winds.
¾ Inverse relation bet. wind effects & water depth.
Variables affecting storm surges:
¾ Winds
¾ Coastline shape
¾ Water depth near the coastline
¾ Storm size and structure
Storm surges are more pronounced:
• over shallow water bodies
• persistent strong winds
• persistent low atmospheric pressure.
coastal flooding many kilometers inland.
Storm surges are particularly pronounced:
• large tidal range areas;
• at high tide;
storm tide: accumulative SL elevation from:
•
•
•
•
storm;
tide;
wave run-up;
freshwater flooding;
storm tide
components
Cause of storm surges
Two distinct types of meteorological disturbances
contribute to major storm surges :
¾ Tropical cyclones
¾ Extra-tropical cyclones
Tropical cyclones – Tropics
Hurricanes – north Atlantic & north-east
(+ south) Pacific Oceans
Typhoons - Northwest Pacific Ocean
Cyclones
- North Indian Ocean
Cyclones
Atmospheric pressure in an offshore area
lower than its surroundings
an area under the influence of a
low pressure weather system.
Low pressure cells + atmospheric lift + strong winds
cyclonic storms.
Sensitivity of coastal regions to cyclones due to warmer
ocean bodies
Dissipation of cyclone energy at the coast
Weaker wind storms over inland areas.
Threat of heavy rainstorms & flooding hazards for the
hinterland.
a. Tropical cyclones:
¾ Large low pressure systems (-100 to + 100 km) intensifying
<> latitudes ~ 5º - 25º.
¾ High SST (>26 ºC).
¾ High relative humidity & evaporation.
¾ Weak vertical wind shear.
¾ Atmospheric instability.
b. Extra-tropical cyclones:
¾ Wind speeds and surge heights are ≤ than in tropical
cyclones but they have a ≥ extent ( >1000 km) and extend
over several tidal cycles.
¾Cause the bulk of flooding and beach erosion in temperate
latitudes.
The Saffir-Simpson scale of hurricane intensity
Category
Central
pressure
(millibars)
Winds
(km/hr)
Surge
(m)
Damage
1
≥980
119 - 153
1.2 – 1.5
Minimal
2
965 - 979
154 - 177
1.8 – 2.4
Moderate
3
945 – 964
178 – 209
2.7 – 3.6
Extensive
4
920 - 944
210 - 249
3.9 – 5.5
Extreme
5
< 920
> 249
> 5.5
Disaster
1. Minimal damage mainly to unanchored mobile homes, threes, shrubs. Minor coastal flooding.
2. Some damage to roofs, windows, mobile homes, piers, vegetation. Coastal flooding; some boats
break moorings.
3. Some structural damage to small buildings; mobile homes destroyed. Flooding destroys coastal
small structures. Floodwater may cover terrain below 1.5m.
4. Major damage to lower floors of coastal buildings. Significant beach erosion. Potential flooding of
terrain below 3m inland as far as 9.6kms; requires mass evacuation.
5. Roofs blown off buildings. Many smaller buildings destroyed or blown away. Damage to lower floors
of buildings below 4.5m elevation within 460m of shoreline. Massive evacuation may be necessary.
Cyclone
structure
¾ At the centre is an ‘eye’ - an area of calm clear sky.
¾ Surrounded by an eye-wall which hosts the strongest winds.
¾ High winds and rain spiral around the eye-wall.
¾ Rotating anti clockwise direction in the N. Hemisphere.
¾ Vice versa in the southern hemisphere.
Hurricane spin
• A system of violent thunderstorms with high winds.
• Circulating about a central low-pressure area.
• Air pressure flows from higher to lower pressure, spiraling
inwards.
• Air does not move in a straight line from high to low
pressure since Earth's surface
spins faster at equator,
slower near poles.
Where do surges occur?
¾ Wherever low pressure systems occur
¾ Intensifying <> latitudes circa 5º - 25º
¾ Onshore blowing winds
¾ In coastal areas, especially with broad, shallow shelves
which funnel.
Where do surges occur?
~ 19.5% of coastal territory of 84 countries vulnerable to
inundation from a 1/100-year storm surge.
A 10% intensification in SLR increases potential inundation
zone to 25.7%.
- an inundation threat for an additional :
¾ 52 million people
¾ 29,164 km2 of agricultural area
¾ 14,991 km2 of urban area
¾ 9% of coastal GDP
¾ 29.9% of wetlands
The Netherlands
¾ 1953: a storm originating in
the North Sea hit
- inundating 150,000 ha
- killing > 2100 people.
1281: > 80,000 deaths
1421: > 100,000 deaths
Largest storm surge
hazards in Europe
Largest storm surge hazards in the world
Bangladesh
1970: surge of >9m; 3 – 500,000 deaths.
1985: ~ 11,000 deaths;
1991: ~ 200,000 deaths.
Bathurst Bay, Australia: 13 - 14.6 m. ; 307 deaths
USA
1900, Galveston: hurricane SLR > 5m killing ~ 6,000 people;
1938, L. Island & s. New England: 2m surge killed >700 people;
2005 Hurricane Katrina: costliest & one of the five deadliest, in
the USA (~ 1,836 deaths).
Mediterranean Hurricanes
¾
¾
¾
¾
¾
¾
¾
September 1947
September 1969
September 1973
August 1976
January 1982
September 1983
December 1984
¾ December 1985
¾ October 1994
¾ January 1995
¾ October 1996
¾ September 1997
¾ December 2005
¾ September 2006
¾ Evidence of a powerful link between rising ocean
temperatures in key hurricane breeding grounds of
Atlantic and Pacific & an increase in the intensity of such
storms.
2004: Cyclone Catarina, South Atlantic
2005: Hurricane Vince, Madeira in Portugal / Spain.
2005: New Orleans overwhelmed by Hurricane Katrina.
2005: Hurricane Rita 4th most intense Atlantic hurricane.
2009: Typhoon Ketsana, Philippines,.
¾ Suggestion that CC induced warming in Med enabling it to
store enough heat to trigger the formation of its own
hurricanes.
Consequences of storm surges
¾ Human loss
¾ Erosion / Flooding
Storm surges account for more hurricane deaths than
any by winds.
Low-income countries susceptible to very significant
damage.
‰ Absent / old urban storm drainage infrastructure
Case study: 136 port cities* investigated for:
- exposure of coastal flooding due to storm surge;
- damage due to high winds in 2070’s;
* (having >1million inhabitants in 2005) Nichols et al (2008).
¾ Asset exposure: > x10 current levels.
¾ Biggest surge impact: Latin America/Caribbean.
¾ Severe coastal GDP losses in East Asia.
¾ Increased exposure (40 – 150 M people) to 1/100 yr
coastal flood event due to:
- climate change;
- subsidence;
- population growth;
- urbanisation;
Storm surge prediction
¾ Good and improving weather forecasts.
¾ US National Weather Service SLOSH model.
¾ Other models.
¾ Problem of dissemination of information.
Case study: Emmanuel et al, (2008)
‰ Storm surge frequency will decrease in S. Hemisphere and
N. Indian Ocean, whilst increasing in the western N.
Pacific.
‰ Changes in tropical cyclone activity is highly influenced by
climate warming.
Mitigating storm surges
¾ Raising public awareness.
¾ Improving meteorological forecasting.
¾ Issues with early warning in low income countries.
¾ Identifying high risk areas and vulnerability.
¾ Develop local / national mitigation plans.
¾ Construction / improvement of coastal defences.
¾ Curbing global warming.
Storm surge mitigation plan
¾ Identify high risk areas prone to storm surges (Storm
surge / vulnerability maps).
¾Collect empirical data needed to obtain return period
curves for flooding and storms particularly for Third
World countries.
¾ Operate storm surge forecasting system.
¾ Improve information and warning dissemination
system.
¾Evacuate high risk areas (e.g. low lying areas)
when storm surge / hurricane is forecasted.
Storm surge maps
‰ Evaluate vulnerability.
‰ Reflect extent of flooding that might be experienced
from various category hurricanes.
‰ Depict worst case, all direction storm scenarios.
‰ Do not always indicate depth of flooding / flooding caused
by heavy rainfall accompanying a hurricane.
Map of vulnerability
of European coasts
to storm surge
hazard
European Spatial
Planning Observation
Network (ESPON)
Long Island South Shore Hurricane Storm Surge Map
http://www2.sunysuffolk.edu/mandias/38hurricane/storm_surge_maps.html
Lessons Learned
from Recent Storm Surge Disasters
¾ Unexpected storm surge & waves larger than the design
possible.
¾ Need to review position of and aged protection measures.
¾ Frequent typhoon attack accumulates the damage.
¾ Insufficient understanding of current design storm water
level.
¾ Uncertainty of return period of current design storm water level.
¾ Uncertainty of future change in the return period due to CC.
C’est tous!
Lessons Learned
from Recent Storm Surge Disasters
1. Coastal defence structures:
a. Poorly positioned:
- beside the section covered
with wave-dissipation blocks
- angle of defence face line
- beach erosion in front of defense.
b. Aged structures:
- surface mortar deterioration
- weed
- Poor materials